Frame beaming/Cab Shake

[Keepontrying]

At about 20,000 miles my cab started to shake/vibrate when hitting rough road, expansion joints and even when hitting the painted cross walk lines on the road. First thing I checked were the cab mount bolts, since I had installed running boards and thought they may have been loose.

 

At 34,000 miles I just replaced the rear cab mount cushions and noticed a significant improvement. Maybe 60=70% of the vibration is gone.

 

Now considering better shocks. Would a set of Monroe reflex or bilsteins help with the ride. Looking for something that will help settle the bumps and smooth the ride out a bit more. I realize it is a 2500 HD truck, but this vibrations should not exist.

 

Also, a local shop does road force balancing and i may go that direction soon also.

 

Any thoughts on the shock choices and road force balancing are appreciated.

 

Ron

 

2009

2500 LT 6.0 litre

34,000 miles

 

[5Alive]

I have more questions than answers, but I'll give the answers you asked for first (thoughts and opinions).

 

Let's say you take your tires and wheels to be "road force" balanced with the Hunter 9700 road force balancer. Immediately after the first tire and wheel assembly is "road force" balanced... immediately after the tire technician removes that first balanced tire and wheel from the 9700's spindle... kindly ask that that same balanced tire and wheel be remounted to the machine to check for balance again. It is IMPORTANT to wait until just AFTER the tire and wheel are REMOVED from the machine, but ask BEFORE the tech mounts the next tire and wheel assembly.

 

He will look at you like you are crazy, but he will be confident to prove to you that the tire and wheel assembly he just balanced will indeed be balanced when he remounts it. I'm not so confident. Unless the specific correct adapter is used for the hub centric wheel in question, and unless more time than usually afforded is taken to mount the tire to the machine, the machine ended up only balancing the specific, idiosyncratic, and most likely unrepeatable way in which the tire and wheel were initially chucked up to the machine. By asking the tech to rechuck the same tire and wheel he just balanced back onto the same machine, you are about to prove this theory.

 

Without a single exception, on the second time around, I have shocked the technician from cockiness to disbelief when the tire and wheel he just balanced a few minutes earlier turns up unbalanced again... not by a little, but by a lot. 4.00 ounces. And this is on the high dollar Hunter 9700 road force balancer. There is nothing wrong with the machine. It is correctly and sensitively reporting the difference between how the tire was chucked up to the machine's spindle the first time, versus how it was chucked up the second time. Here is the key: Those time efficient "universal" taper cone chucks do not do the hub centric wheels justice. The problem is, for a tire shop to order all the specific adapters to every wheel is very expensive tooling to have on hand, and very time consuming and training intensive to get employees to do.

 

So, if your tire technician proves to be the single exception to the rule that I have found in my experience... proved by the first tire and wheel showing 0.00 on both sides after having been REMOVED completely from the machine, and then remounted to the machine and tested again, then by all means proceed and have that tire technician complete the other three tire and wheel assemblies. If, as I suspect, you find that the remount of the first tire and wheel does not repeat the balanced state it was in before, then you'll have to decide whether or not it is worth the additional expense to continue the exercise of "balanced" in name, but not in deed.

 

Obviously, the way the tire and wheel chuck up to the truck will be different yet again from the way it is chucked up to the machine. One (expensive) way to achieve dynamic balance of the entire rotating assembly the way the wheel is chucked to the truck, along with balancing the brake rotor, hub, and indeed the entire rotating assembly, including the mud, snow, and rocks stuck in the tire grooves, is with Balance Masters (encapsulated mercury, much quieter) or Centramatic (shot beads in oil, louder, but no mercury) dynamic balancers mounted between the wheel and the rotor hub flange. I have these on my trucks (both brands, depending on wheel type).

 

So those are my thoughts and opinions. Now on to my questions:

 

How did you determine that your REAR cab mounts needed to be replaced? Was there a visual indication? Did you just start with the pair that was easiest to access? Were you attempting to swap the cab mounts furthest away from both the front and the rear axles, where the frame would have the greatest deviation from the plane established by either axle, subject to the limitation of spring travel? What inspired you to pick the rear cab mounts, instead of say, the middle cab mounts?

 

If you experienced a 60% to 70% improvement after replacing the rear cab mounts, why weren't you tempted to continue in that successful direction and replace the rest of the cab mounts, instead of moving to other things like wheel & tire balance, or shocks? What made you decide that just the rear cab mounts were "enough."?

 

When you removed the rear cab mounts, did you notice (or measure, but I doubt anyone measures disassembly torque outside of FEA) how tight or loose they were? You mentioned the first thing that you checked were the cab mount bolts. How did you "check" them? What were you checking for? Bolt torque? Cushion compression? Assuming that in the process of replacing the rear cab mounts with new pieces, you tightened the rear cab mounts to the assembly torque called for in the book, is it possible that if you tightened the existing cushions to the same torque, that the improvement you noticed would be the same?

 

These are all genuine, not loaded questions. I do not know the answers to any of them, and have no guesses as to what the answers should be or lead to. But I have always been curious about the influence of cab mount durometers and torque in mitigating road imperfections, and you are the first person I've read of who noticed such a large improvement in ride quality from replacing cab mounts that had less than 36,000 miles on them.

 

I have heard of cab mounts getting replaced from creaking and clicking, where the threads of the thru bolt are washboarding over a horizontal flange in the assembly. But your situation is different, and I'm very curious about it.